It has previously been suggested in the aforesaid Becker et al application to feed envelopes from the bottom of a stack through a nip, which is formed between a feed roll and a restraint roll and receives a leading edge of each of the envelopes to be advanced. The restraint roll is rotated in the same direction as the feed roll so that the surfaces at the nip formed by the two rolls move in opposite linear directions. By forming the feed roll of a slightly higher coefficient of friction to paper than the restraint roll, the feed roll has a greater tangential drive than the restraint roll so that the lowermost envelope is advanced from the stack.
A torque limiting clutch has the biasing force of its spring selected so that the restraint roll rotates with the feed roll since the torque resulting from the tangential frictional force at the surface of the feed roll is greater than that produced by the restraint roll due to the torque limiting clutch. However, this biasing force is not so large as to cause rotation of the restraint roll with the feed roll when more than one of the envelopes enters the nip formed between the rolls so as to prevent more than one envelope from passing through the nip.
While the envelope feeder of the aforesaid Becker et al application has satisfactorily fed most envelopes, it has been found that envelopes of relatively long length (for example, greater than 12 inches) create a drag. It also has been found that envelopes with flaps of certain configurations tend to conform to an adjacent envelope so that two of these envelopes are advanced simultaneously from the bottom of the stack but cannot enter the nip because of their total thickness. Generally, envelopes vary widely in form, texture, and bundling throughout the world, resulting in various tendencies to feed unreliably.